This invention relates to a protective coating composition. More particularly, it relates to a silicone resin coating composition which, when applied to a substrate, forms a protective, abrasion-resistant and discoloration-resistant coating thereon.
Recently, the substitution of glass glazing with transparent materials which do not shatter or are more resistant to shattering than glass, has become widespread. For example, transparent glazing made from synthetic organic polymers is now utilized in public transportation vehicles, such as trains, buses, taxis and airplanes. Lenses, such as for eye glasses and other optical instruments, as well as glazing for large buildings, also employ shatter-resistant, transparent plastics. The lighter weight of these plastics in comparison to glass is a further advantage, especially in the transportation industry where the weight of the vehicle is a major factor in its fuel economy.
While transparent plastics provide the major advantage of being more resistant to shattering and lighter than glass, a serious drawback lies in the ease with which these plastics mar and scratch, due to everyday contact with abrasives, such as dust, cleaning equipment and/or ordinary weathering. Continuous scratching and marring results in impaired visibility and poor aesthetics, and oftentimes requires replacement of the glazing or lens or the like. Another shortcoming of some transparent plastics is that under continuous exposure to ultraviolet light the surface of the plastic will eventually discolor, e.g., yellow, as the result of a chemical breakdown of the polymer structure.
One of the most promising and widely used transparent plastics for glazing is polycarbonate, such as that known as Lexan.RTM., sold by General Electric Company. It is a tough material, having high impact strength, high heat deflection temperature, good dimensional stability, as well as being self-extinguishing, and is easily fabricated. Polycarbonate, however, is particularly sensitive to discoloration from ultraviolet light exposure.
Attempts have been made to improve the abrasion resistance of transparent plastics. For example, scratch-resistant coatings formed from mixtures of silica, such as colloidal silica or silica gel, and hydrolyzable silanes in a hydrolysis medium, such as alcohol and water, are known. U.S. Pat. Nos. 3,708,225, 3,986,997 and 3,976,497, for example, describe such compositions.
Copending U.S. application Ser. No. 964,910, filed Nov. 30, 1978, discloses another abrasion-resistant coating composition. This coating composition has been found to be highly desirable as a protective finish for plastic, as well as metal or metallized substrates. A particularly significant area of application for this coating is in the glazing and optical lens industry. Since these areas require a coating of high optical clarity, coatings which show flowmarks, dirtmarks or other marks which may impair visibility, are undesirable. In copending U.S. application Ser. No. 964,911, filed on Nov. 30, 1978, it is disclosed that the addition of a small amount of a polysiloxane polyether copolymer to the coating compositions disclosed therein eliminates the occurrence of undesirable flowmarks and the like, as well as providing other improvements in the hard resistant coating.
It has now been discovered that if certain ultraviolet light absorbing compounds are incorporated into the coating compositions disclosed and claimed in said copending U.S. Ser. No. 964,911, and these coating compositions are applied to the surface of a plastic substrate, such as polycarbonate, not only is an excellent mar resistant coating provided but the service life of the substrate is even more extended by the prevention of discoloration and delamination which is usually associated with these substrates.